1. Field of the Invention
The present invention relates to a color management system for transforming source color data in a CMYK source (cyan, magenta, yellow and black) color space into destination color data in a CMYK destination color space, wherein the color management system exempts CMYK color data from the transformation sequence if the C, M and Y values of the CMYK color data are each equal to zero, regardless of the K value. In such a case, the CMYK color data is passed directly to the destination color data without modification.
2. Description of the Related Art
Color management systems are used for transforming color image data from the color space of an input device, such as a digital camera or scanner, to the color space of an output device, such as a printer or monitor. Generally, in color management systems such as the system of the International Color Consortium (ICC), the component colors of source color data are first transformed from a device-dependent color space for the source device into a device-independent profile connection space (PCS), such as CIELAB color space or a perceptual color space. The color data is then transformed from the PCS color space into a device-dependent color space for the destination device. For example, RGB color data from a color monitor is transformed into CIELAB color space and is then transformed from the CIELAB color space into the destination CMYK color space for a color printer.
Such color management systems generally produce desirable results and are able to match colors accurately when viewed on, or acquired by, different devices. However, many times, particularly in color proofing operations, it is desirable to transform color image data from a source CMYK color space to a destination CMYK color space, such as from the color space of one type of color printer to the color space of another type of color printer. In such situations, it may be desirable to avoid performing color management on portions of the source CMYK color data that have no color components.
For example, a portion of an image may be comprised only of black text or black lines. The portion may be represented in the source CMYK data by using only the K component, and not the C, M or Y components. For example, it is often desirable to instruct a printer to use only black ink to render a portion of an image which contains only black text or black lines. Normal processing of such an image portion by a color management system may result in some or all of the portion being represented in the CMYK destination color data by the C, M and/or Y components; not just the K component. This is because there are many possible CMYK combinations that a color management system may utilize in an attempt to reproduce the appearance of the original black. For example, some black inks have a slight color hue, such as blue or red. In such a case, a color management system may attempt to recreate the appearance of the slight color hue of the black ink by adding a C, M and/or Y component, instead of using just K. It should be appreciated that the combination necessary to achieve an acceptable black appearance is very delicate, and if the combination is not correct, a brownish, muddy appearance can result. Furthermore, the printing of black text and/or other objects with fine detail using multiple colors of inks can result in blurred text and/or objects due to slight errors in alignment while printing.
The transformation of the K component of CMYK source color data into the C, M and Y components of CMYK destination color data can occur when the color management system being used applies a different technique to represent black image portions than that of the color system which created the CMYK source color data. For example, the color system that generated the CMYK source color data may have used a particular black generation algorithm, such as undercolor removal, to represent the black image portions, but the color management system used to generate the CMYK destination data may apply a different black generation algorithm, such as gray component replacement. Such a color management system will transform portions of the CMYK source color data that contain zero values for the C, M and Y components into CMYK destination color data that contain non-zero values for the C, M and Y components. As discussed above, this can result in an undesirable appearance of black text and lines which were intended to be rendered by black ink (K) only.
Thus, existing color management systems often result in a problematic appearance of the image rendered on the destination device due to the introduction of an undesirable color cast and of unwanted color components when converting from CMYK source color data to CMYK destination color data. This can cause portions of the image that were meant to be achromatic to have an undesirable chromatic appearance.